Drying method



96 BPL IN TAILINGS Aug. '12, 1958 2,847,123

J. E. LAWVER DRYING METHOD Filed Sept. 1, 1953 f gl %BPL IN TAILINGS %BPL IN CONCENTRATE ZBPL IN TAILINGS u .1

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6 8 PL IN CONCENTRATE ATTflRNEX United States Patent DRYING METHOD James E. Lawver, Lakeland, Fla., assignor to International Minerals & Chemical Corporation, a corporation of New York Application September 1, 1953, Serial No. 377,779 9 Claims. (Cl. 209-127) This invention relates to the art of electrostatic separation of soft readily attritionable ores. More particularly it relates to the preparation of dry substantially slimefreepulverulent soft material which is amenable to electrostatic separation. Still more particularly the invention relates to a method of drying soft material so that when utilizing the phenomenon of contact potential the ore particles will accept differential charges and thus respond to electrostatic separation when passed through an electrostatic field in a freely falling state.

It is well known that materials exhibiting differential electrical effects, even though electrically rated as nonconductors or insulators, may be separated by electrostatic .methods. The Patent to OBrien 2,168,681 shows several forms of apparatus by means of which particles of we may be separated by subjecting them as freely falling bodies to the action of an electrostatic field.

A prerequisite to the separation of the particles is that they be differentially charged by contact with a substance with a work function different from the particles such as a grounded plate of suitable material having a work function different from that of the ore, or by intimate contact with unlike particles over a substantial area under conditions where surface conductivity will not dissipate .the electrical charges.

Drying of hard ores, i. e., those which do not readily form fines or slimes upon being subjected to agitation or to attrition action, apparently has little effect detrimental to the charging and separation actions. Soft ores, on the. other hand, such as Florida phosphate upon drying under agitation conditions, particularly when drying or when approaching dryness may be adversely affected to a greater or a lesser degree depending upon the violence and upon the length of time of agitation. Agitation of such soft ores when substantially dry, if of sufficient duration and/or of sutficient violence, can render an ore substantially unresponsive to electrostatic field separation.

This adverse effect of agitation creates a problem of the first magnitude in the design and operation of a commercial plant for dry beneficiation of soft ore, because to be profitable tremendous quantities of a relatively low profit margin material must be processed. The cost of equipment such as ovens for processing 500 tons per hour of soft ore Without agitation (comparable in amount to floatation plant operation), would be prohibitive since the heat transfer coefficient under such conditions is usually less than 10 B. t. u./hour/square foot/log mean temperature difference. On the other hand, in systems where a high heat transfer is attainable for treatment of a large volume of material, the agitation has in the past rendered such systems substantially use less in preparation of certain phosphates for electrostatic separation.

It is an object, therefore, of this invention to overcome the disadvantages and shortcomings of normaldrying methods heretofore in use in the phosphate industry.

It is another object of this invention to provide a drying method for phosphates which minimizes agitation effects.

It is still another object of this invention to provide a method which reduces markedly the time during which the dried ore is subject to agitation.

It is still a further object of this invention to provide a method of drying which does not produce a dry ore feed contaminated with deleterious quantities of attrition form-ed secondary slimes.

These and other objects of the invention parent to those skilled in the hereinafter presented.

It has been discovered that the deleterious effects of agitation can be minimized so that the lack of response of relatively soft and attritionable non-metallic comminuted ore to the forces in an electrostatic field can be cured when heating the ore to dryness under agitating conditions, provided heat transfer is maintained high and the time of agitation is minimized.

It will, of course, be recognized that the simplest drying method from the point of view of electrostatic will be apart from the description separation as taught by Cook in the copending application Serial No. 277,226, filed March 18, 1952, now abandoned in favor of continuing application Serial Number 454,524, filed September 7, 1954, by the same inventor, is one where the agitation is zero as the material approaches dryness. Under such conditions the heating or drying time is immaterial. However, in certain operations such method may be uneconomical and in the instant drying method it has been discovered that nearly equal results are attained while agitating the material by increasing the heat transfer and balancing the time of agitation against the degree of agitation. Preferably, the drying is carried out in at least two stages, in the first of which thebalance of degree of agitation against time of agitation is not as critical as in the second stage because the moisture content is still high.

In the final stage of the multi-stage drying, according to this invention, the material is agitated and the agitation inversely balanced against time of agitation, i. e., the greater the agitation, the shorter the permissible agitation time. In this manner damage to the separation characteristics can be reduced to substantially zero judged by comparison with separations obtainable with materials dried by other methods.

The separation process in its preferred form consists in subjecting deslimed ore, in a first drying stage, to the drying effects of suspension in hot gases having a mass velocity such that the largest particles will be conveyed vertically, if the apparatus is so designed, and not allowed to remain in a teeteringor fluidized bed condition. When ore of particle size in the range of about 14 mesh to about 200 mesh and having a moisture content of about 8% is moved in gases having a minimum mass velocity of about pounds of gas/minute/ square foot of bed area, i. e., 2500 cubic feet per minute (70) air in a 14 inch diameter pipe at an inlet gas temperature of about 2500" F., the solids after a short vertical travel of about 20 feet into a cyclone separator can be controlled to have a temperature of about 160 F. to about F. The heat transfer coefficient calculated for this described operation is v9400 B. t. u./hour/square foot/log means temperature difference. Solids at this temperature range having been held at such a temperature for only a short time will have a moisture content of about 0.5% to about 0.8%. Moisture content at this level is sufficiently high to have deleterious effects upon the electrostatic separation characteristics of the ore.

Solids from the foregoing first drying stage may have the drying completed in a final drying stage in a variety of apparatus provided the degree of agitation is counterbalanced by shortening of the time of agitation. To

illustrate with but two examples, the Nichols-Herreschoff hearth in its normal arrangement of six hearths in series and having a heat transfer coefficient of about 6 B. t. u./ hour/square foot/log mean temperature difference agitates the material by raking it to such a degree that after raking over the 6 hearths, electrostatic separations of the ore is reduced below economic limits. If the hearths are arranged in parallel in this apparatus so that material is raked only over one or two hearths and the hot air system reconstructed to obtain a heat transfer coefficient in the range of about 25 to about 50 B. t. u./hour/square foot/log mean temperature difference, the agitation is not sufiicient to deleteriously effect the electrostatic separation. This particular type of apparatus is of a type affording minimum agitation. The range of heat transfer coefficient useful in the instant separations will vary from about 25 to about 12,000 E. t. u./hour/square foot/ log mean temperature difference.

When the agitation is violent, the time of agitation must be kept minutely short. With this in mind the final stage drying can also be carried out in a conveying type drier provided the agitation time or retention time is reduced, for example, to about one-fifth what it would be in the first stage conveying drier. By way of comparison, if the retention time for unsized feed of a particle size in the range of -14 +150 mesh in the first stage drier at a mass velocity for drying gases of 4800 pounds/hour/ square foot is 2.8 seconds, then at a mass velocity of 9600 pounds/hour/square foot the retention time will be of the order of 0.45 second.

At the end of the final drying stage, the solids should be at a sufficiently low moisture content that separation is not deleteriously affected. One way to be sure that the moisture content of the comminuted solids is sufficiently low is to bring the solids out of the final drying stage at a temperature in excess of about 300 F. This insures that in the short drying period the moisture content of the phosphate material will be reduced below about 0.2% and in all probability below 0.1%.

It is not to be inferred from the discussion of multistage drying that the above described methods are the only way in which phosphate ores can be dried satisfactorily. Economics dictates the choice of the preferred two stage operating method. Florida phosphate ore can be dried in a single stage of conveying drying provided the mass velocity is high enough and the heat transfer in the system is suificient. When utilizing gas at a conveying gas velocity of the order of 3050 feet per minute (70), the gas having initial temperatures of about 2200 F. and a conveying gas temperature of about 350 P., the phosphate solids can be removed from the collection cyclone at a temperature in the range between about 275 F. and about 375 F., optimum temperature being about 310 F. The retention time for the solids under these conditions would be of the order of 0.25 second. Material dried in a single pass under these conditions gives excellent separation upon cooling to an optimum separation temperature and passage through electrostatic fields.

When the material to be separated is thoroughly dry on the surface, the solids are cooled to a temperature range for optimum separation results, said temperature being subject to variation depending upon the moisture content of the solids and the moisture conditions under which the solids are being cooled. At high relative humidity the optimum temperatures for separation will vary from about 125 F. to about 300 F. with a preferred range of about 175 F. to about 235 F. At such temperatures, the vapor pressure relationships are such that the particles do not rapidly adsorb or take on moisture. If the particles are held under controlled humidity conditions, the solids can be cooled to atmospheric temperature or below and still give excellent electrostatic separations. Separations have been made with solids with temperatures of 0 F. and 50 F. with the separations showing only slight variation from those accomplished using temperatures of 200 F.

Alternatively, if the surfaces of the ore particles are thoroughly dry while internal moisture is present therein, and if the rate of diffusion of such internal moisture be sufficiently slow, such surface-dry condition, if maintained until after charging and displacement of the ore particles, will permit effective separation of the particles.

In order to electrostatically separate the components of an ore it is necessary to grind to economic liberation. Economic liberation may be accomplished for Florida phosphate rock by grinding to less than about 14 mesh size or by sizing out the l4 mesh material in the matrix which is the common practice. Particles of the comminuted phosphate ore generally fall in the range between about l4 mesh and about 200 mesh, with the feed material for the electrostatic separation preferably having a particle size in the range of about -24 mesh to about +150 mesh. Unsized feed, however, can be handled by this system and materials having particle size of +1 millimeter to about microns have been effectively separated.

After the comminuted material is reduced to the desired low surface moisture content, the particles are induced to accept an electric charge. As distinguished from other methods in common use (where, for example, material is not merely polarized as in the case of pyroelectric crystals) the charging of the particles may be and preferably is carried out in the absence of an electrical field. In the performance of this step, the particles are caused to contact one another or to contact a conductor or donor surface connected to a surce of free electrons as by being grounded to the earth by an electrical conductor. One method of accomplishing this result is to convey, for example, phosphate ore particles having a temperature in the range of approximately 200 F., to approximately 325 F., to a feeder having a grounded contact surface of lead, zinc, aluminum, copper, tin, iron, and the like. The particles are caused to fiow in a thin stream over the chute surface which is connected by an electrical conductor to the earth. Such donor surfaces may be a grounded metallic chute, plate tray, hopper or similar device.

Particles which have acquired a charge may then be separated as, for example, by being fed as freely falling bodies between the electrodes of one or more electrostatic separating units; i. e., in a path normally not in contact with said electrodes.

The strength of the electrostatic field maintained between electrodes Which will effectively alter the path of falling particles, varies with the particle size of the ore fed to the separator. The voltage may vary from 3,000 volts per inch of distance between electrodes in separating material of relatively fine particle size in the range of approximately mesh to approximately 200 mesh, to 15,000 volts per inch of distance separating electrodes handling coarse particles. In all such discussions of field strength, it must be borne in mind that corona discharges which ionize air are to be avoided. In general, it is preferred to operate with a total impressed difference in potential of about 70,000 to about 90,000 volts, although voltages as low as about 20,000 and as high as about 200,000 are utilized on occasion. This voltage should be maintained at a high direct voltage potential substantially free of alternating current components; i. e., filtered D. C. current should be low in the so-called A. C. ripple. A standard supply of D. C. voltage may also be obtained without expensive filtering apparatus by the use of such equipment as radio frequency power supply.

The process for seperating ore into its components will be more fully understood from the following description given by way of example with reference to the separation of phosphate ore.

The precise nature of the invention will be more Example I Florida phosphate pebble obtained as washer debris having by screen anlysis particles in-the size range of -24 +100 mesh was subjected to intensive water washing and scrubbing after the normal phosphate -desliming operation. This scrubbing operation was performed in an agitating unit at 70% solids and the scrubbed washer debris was dewatered in a Hardinge dragclassifier.

The solids or comminuted ore from the classifier had a water content of about 20%. This *wet ore was stored until the ore had drained to a moisture content of about 7%%. A portion of the comminuted ore was heated to a solids temperature of approximately 250 F. in an electric oven. Samples were withdrawn for. electrostatic separation at zero agitation, 2 minutes agitation, 5 'minutes agitationand 10 minutesagitation (all with an Ajax- I.oveyor). The samples were agitated in a metal pan and each sample run in an-electrostaticunit operating as follows: The samples as feedwere delivered to a Syntron vibrating trough which had the metal trough thereof grounded to the earth by an electrical conductor. The Syntron trough discharged the particles as freely falling bodies between electrodes spaced approximately 10 inches apart and maintained at a potential gradient of approximately 8000 volts per inch. The rate of feed of the dried comminuted material was approximately one ton/hour/ linear foot of electrode breadth. Temperaturebf the material as it passed through the electrostatic field was approximately 200 F. in each instance.

Curves Al, A2, A3, and A4 of Figure 1,'respectively, are the curves for zero, 2, 5 and '10 minute agitation, respectively, and show the reduced effectiveness of elec trostatic separationas the phosphate 'is agitated when dry.

Example II Deslimed Florida phosphate pebble obtained as washer debris having a screen analysis in the size range of 24 +100 mesh was washed and scrubbed in the same manner as the feed to Example I. The drained solids were fed to a conveying type drier in which the velocity of the hot gas was approximately 1440 feet per minute or approximately 3 times the velocity necessary to fluidize a bed of the largest particles in the ore feed. The solids were conveyed vertically approximately 20 feet and recovered from a cyclone separator at a temperature .of approximately 195 F. The final moisture was removed from this partially dried ore in a second heating stage by heating in an electric oven to a solids temperature of approximately 300 F. The dried material was then passed through the electrostatic separation unit described in Example I which apparatus was maintained under conditions substantially identical with those maintained in Example I. Curve B of Figure 2 shows the average results of three separations of ore dried as above described.

Similar feed material was then dried in a first drying stage in the conveying drier under conditions whereby the air velocity was about 3050 feet per minute and the retention time of the solids in the conveying drier was 0.45 second as compared to 2.8 seconds of the above described run. Material was removed from the conveying drier at a temperature of approximately 200 F. and

dried in a seconddrying stage in the electric oven to a solids temperature approximately 300 'F. in the same manner as previously described. Curve C of Figure 2 shows the average results of six separation tests as above described.

In order to have a comparison, feed material was also dried in the oven alone. Curve D shows the average results of separations made upon four examples of material dried solely in the electric oven. Clearly the low retention time of a 0.45 second in the conveying drier gave results substantially equal to those obtained with material which was oven dried and shows a marked improvement 'in the bone phosphate of lime content 'of the concentrate 'over the material agitated for a longer time 'in the conveying type drier.

Example III Florida phosphate pebble similar to that described in connection with the work of Example I was scrubbed to complete the desliming. When the moisture content of the stored comminuted ore had drained to about 8%, the solids were fed to a conveying type drier in which the velocity of the hot gas was approximately 1440 feet per minute and three times the velocity necessary to fluidize a bed of the largest particles in the ore feed. The solids were conveyed vertically approximately 20 feet and recovered from a cyclone separator operated at a temperatureof approximately F. the retention time in the conveying drier was approximately 2.8 seconds. This partially-dried ore having a moisture content in the neighborhood of 1% was fed to a Nichols-Herreschoff drier of six hearths. The material when dry was passed through the electrostatic separation unit utilized for the separations in Examples I and II. Separation results obtained in this test are shown in curve B of Figure 3. The Nichols-'Herreschoff drier was next rearranged so that material was fed only onto one hearth. The air flow system was reconstructed so that gases circulated over the one hearth at an inlet temperature of approximately l800 F. Solids were removed from this hearth at approximately the same temperature; namely, about 285 F., as the temperature of the solids has been when the material was passed over six hearths. This material was separated in the *same electrostatic unit as the previously dried material and theseparation effected is shown in "ourveF of Figure '3. Curve F approximates the separation made with the-above dried material and curve B clearly illustrates the deleterious effects upon separation o'ftheadditional raking of the ore duringits passage over the six hearths.

' Example IV Deslimed scrubbed ore was dried in an oven and sep arately. in the 14 inch diameter conveying drier as was utilized .for drying stage 1 of :Examples II and III.

The solids. first were dried in .a single stage of conveying drying .in gas having a velocity of 3050 feet .per minute (70 F.) and collected at a solids temperature ofapproximately 350 F. This dried material was cooled to 225 F. .and separated in the electrostatic apparatus described in-connection with the other examples. Curves G .and .H of .Figure 3 show the results of separation of the oven dried and single .conveyingstage dried material respectively.

The partiallydried :solids having a temperature .of approximately 195 F. were delivered to a second conveying drier. This conveyingtdrier was supplied with hot gas at a temperature of approximately 2200 F. The gas had a velocity of approximately 3050 feet per minute giving a solids retention time in the drier of approximately'0.45 second. Material collected from the separation cyclone had a temperature of approximately 350 :F. This material was passed through the electrostatic separation unit utilized in Examples II and III. Shortening of t'he :tirneof agitation-in a second drying stage improves the character of the separation.

tering condition for the largest particles being dried, re-

moving the partially dried solids from suspension at a temperature in the range of 175 F. to about 210 F., and subjecting the partially dried particles in a final stage of moisture removal to drying gases under conditions. giving a time of agitation in the range of about 0.1 second to about minutes, the shorter periods of agitation being utilized when the agitation is greatest, and a heat transfer permitting attainment of a solids temperature in' excess of about 250 F. during the period of agitation electrically charging the dried ore particles and subjecting the charged dry ore to an electrostatic separation to beneficiate the same.

2. The method of beneficiating soft attritionable phosphate ore which comprises subjecting the washed comminuted ore to drying gases under agitation conditions in a first stage of drying where the solids are moved by hot drying gases having a velocity in excess of that for maintaining a teetering condition for the largest particles being dried, removing the partially dried solids from suspension at a temperature in the range of 175 F. to about 210 F., and subjecting the partially dried particles in a final stage of moisture removal to drying gases under conditions giving a time of agitation in the range of about 1 minute to about 15 minutes, the shorter periods of agitation being utilized when agitation is the greatest, and a heat transfer permitting attainment of a solids temperature in excess of about 250 F. during the period of agitation, inducing the' dried particles to accept an electrical charge, subjecting the charged particles as freely falling bodies to the attracting and repulsing forces of a high potential electrostatic field, and recovering a product of high phosphate content.

3. The method of beneficiating soft attritionable phosphate ore which comprises subjecting the washed comminuted ore to suspension in drying gases under conditions providing a velocity of gases in excess of that for maintaining a teetering condition for the largest particles being dried and giving a time of suspension in the range of about 0.1 second to about 2 seconds and a final dry solids temperature in excess of about 300 F., the inlet temperature of the gas being in the range of about l,800 F. to about 3,000 F., inducing the dried particles to accept an electrical charge, subjecting the charged particles as freely falling bodies to the attracting and repulsing forces of a high potential electrostatic field, and recovering a product of high phosphate content.

4. The method of beneficiating soft attritionable phosphate ores which comprises washing deslimed comminuted ore free of secondary slimes, conveying the washed ore in hot drying gas suspension to a collection zone from which the particles are recovered at a temperature of about 350 F., the hot gases having an initial temperature of about 2,500 F., and the time of conveyance being of the order of 0.15 second, inducing the dried particles to accept an electrical charge, subjecting the charged particles as freely falling bodies to the attracting and repulsing forces of an electrostatic field, and recovering a product of high phosphate content.

5. The method of beneficiating phosphate pebble ore which comprises washing deslimed comminuted ore free of secondary slimes, conveying the washed ore in a primary drying stage in hot drying gas suspension to a collection zone from which the partially dried particles are recovered at a temperature of about 195 F., the hot gases having initial temperatures of about 2,500 F. and average time of conveyance of particles being about 2.8 seconds, completing the moisture removal by passage of gas having initial temperature of about 1,200 F. over said partially dried particles as the material is slowly raked at a rate giving a retention time in the secondary drier of about 5 minutes during which time the solids are raised to a temperature of about 300 F., cooling the particles, inducing the particles to accept an electrical charge, subjecting the charged particles as freely falling bodies to the attracting and repulsing forces of an electrostatic field, and recovering a product of high phosphate content.

6. The method of beneficiating phosphate pebble ore which comprises washing deslimed comminuted ore free of secondary slimes, conveying the washed ore in a primary drying step in hot drying gas suspension to a collection zone from which the partially dried particles are recovered at a temperature of about 195 F., the hot gases having initial temperatures of about 2500 F. and the average time of conveyance of the particles being about 2.8 seconds, completing the moisture removal by resuspension of the particles and conveyance to a second collection zone from which the dried particles are recovered at a temperature of about 325 F., the temperatures of the final drying gases having initial temperatures of about 2500 F. and the time of conveyance being about 0.45 second, cooling the solid particles to about 200 F., inducing the particles to accept an electrical charge, subjecting the charged particles to the attracting and repulsing forces of an electrostatic field, and recovering a product of high phosphate content.

7. The method of preparing relatively soft attritionable non-metallic ore for electrostatic separation which comprises suspending and heating liberated ore particles, while agitating, in hot drying gases to a solids temperature in excess of 200 F., the heat transfer coeificient for drying being in the range of between about 25 and about 12,000, the hot drying gases used for at least the initial heating of the ore particles having a velocity in excess of that for maintaining a teetering condition for the largest particles being dried, the agitation While heating being for a time in the range of between about 0.1 second and about 15 minutes, the shorter period of agitation being utilized when the degree of agitation is greatest electrically charging the dried ore particles and subjecting the charged dry ore to an electrostatic separation to beneficiate the same.

8. The method of preparing relatively soft attritionable non-metallic ore for electrostatic separation which comprises suspending and heating liberated ore particles in hot drying gases to a moisture content of less than 0.2% by weight when utilizing the heat transfer coeflicient in the range of between about 25 and about 12,000, the hot drying gases used for at least the initial heating of the ore particles having a velocity in excess of that for maintaining a teetering condition for the largest particles being dried, the agitation while heating being for a time in the range of between about 0.1 second and about 15 minutes, the shorter period of agitation being utilized when the degree of agitation is greatest electrically charging the dried ore particles and subjecting the charged dry ore to an electrostatic separation to beneficiate the same.

9. The method of preparing relatively soft attritionable non-metallic ore for electrostatic separation which comprises suspending and heating liberated ore particles in hot drying gases in at least two drying stages to a dry solids temperatures in the range of between about F. and about 210 F., the hot drying gases being used to move the particles in at least the first stage and having a velocity in excess of that for maintaining a teetering condition for the largest particles being dried, the time of agitation being in the range of between about 0.1 second and about 1 minute, the shortest period of agitation being utilized when agitation is the greatest and in another stage of drying reducing the solids to final dryness while in suspension in gases having a temperature in the range between about 1500 and about 3000" F. at the inlet to the contacting zone, the hot gases moving the particles having a velocity in excess of that for maintaining a teetering condition for the largest particles being dried and the time of retention in suspension being in the range between about 0.1 seconds and about 10 seconds electrically charging the dried ore particles and subjecting the charged dry ore to an electrostatic separation to beneficiate the same.

References Cited in the file of this patent UNITED STATES PATENTS 

1. THE METHOD OF PREPARING RELATIVELY SOFT AND ATTRITIONABLE NONMETALLIC ORE FOR ELECTROSTATIC SEPARATION WHICH COMPRISES SUBJECTING THE WASHED, COMMINUTED ORE TO DRYING GAS UNDER AGITATION CONDITIONS IN A FIRST STAGE OF DRYING WHERE THE SOLIDS ARE MOVED BY HOT DRYING GASES HAVING A VELOCITY IN EXCESS OF THAT FOR MAINTAINING A TEETERING CONDITION FOR THE LARGEST PARTICLES BEING DRIED, REMOVING THE PARTIALLY DRIED SOLIDS FROM SUSPENSION AT A TEMPERATURE IN THE RANGE OF 175*F. TO ABOUT 210*F., AND SUBJECTING THE PARTIALLY DRIED PARTICLES IN A FINAL STAGE OF MOISTURE REMOVAL TO DRYING GASES UNDER CONDITIONS GIVING A TIME OF AGITATION IN THE RANGE OF ABOUT 0.1 SECOND TO ABOUT 15 MINUTES, THE SHORTER PERIODS OF AGITATION BEING UTILIZED WHEN THE AGITATION IS GREATEST, AND A HEAT TRANSFER PERMITTING ATTAINMENT OF A SOLIDS TEMPERATURE IN EXCESS OF ABOUT 250*F. DURING THE PERIOD OF AGITATION ELECTRICALLY CHARGING THE DRIED ORE PARTICLES AND SUBJECTING THE CHARGED DRY ORE TO AN ELECTROSTATIC SEPARATION TO BENEFICIATE THE SAME. 